Strain improvement of industrial microorganisms is required at both the clinical testing stage and commercialization stage of drug development. Although strain improvement plays an enormous economic role in reducing the costs of pharmaceutical manufacturing, this role is generally unrecognized by the public, except during a national crisis when demand for a drug may soar. The first such crisis involved the production of penicillin during World War II. Future crises may involve drugs needed to counter bio-terrorist attacks. Unfortunately, the empirical genetic methods used for strain improvement have not substantially improved since the 1940's. Modern genetic engineering technology offers hope that newer rational methods can be developed, but more basic research is required. This research is no longer being rigorously pursued by the pharmaceutical industry. In Phase I of this project we significantly broadened our understanding of the genetics of erythromycin production by cloning and sequencing a new erythromycin gene cluster. In Phase II we are proposing to apply this new basic knowledge to the development of modern methods of strain improvement for erythromycin and other important drugs.